This invention is concerned generally with an electronic blasting system and more particularly is concerned with a process whereby a detonator or a series of detonators may be rendered safe regardless of the state of the blasting system or of the integrity of a communications system which is used in the blasting system.
A blasting system usually incorporates means for testing the wiring in the system and connections between the detonators and a blast controller. During a testing phase and also during a programming phase power must be applied to one or more of the detonators, an operation which raises the risk of an unintended event such as a blast. The risk is increased if one or more detonators are in an armed state and a need to abort the blast arises. For example a detonator could remain in a armed state and not respond to a disarm signal if there is a poor connection in a communication system which is used in the blasting system, if a detonator is intermittently faulty, if a cable is damaged, due to the ingress of moisture or for any other reason which interferes with communication between one or more detonators in the system, and a blast controller.
If a detonator does not disarm, despite the transmission of a disarm signal, eg. from a blast controller, then the detonator can remain in the armed state for many hours and, if reconnected to a blasting system, the detonator will remain armed, a condition which could result in an unintended blast.
It is also practice, when a disarm mode is required, to wait a predetermined time period to allow energy which is stored at each detonator to dissipate to a level which is low enough to ensure that initiation of an explosive cannot take place. The energy at each detonator is normally stored in a capacitor and as the capacitor discharge is exponential it can be necessary to wait for a considerable period. If however energy discharge takes place along a path which is defective or damaged then it cannot be said with certainty that, after a predetermined time period, the energy level at the detonator is sufficiently low to render it safe. An allied factor is that electronic components and circuits which are associated with the detonator might not function satisfactorily, due to a low voltage supply, and settings of the detonator might be lost, creating an undefined and unsafe condition.